Telecommunication systems are required to provide data transfer between a large number of data paths through a central or distributed processing system. Communication system requirements of flexibility, power, speed of interface and a number of electrical signals necessary to implement such an interface are limited by practical physical and processing limitations such as to the number of signals for each data path to be used for a data transfer, as well as the cost of implementation of the large number of signal paths which need to provided. The number of address signals and corresponding hardware can be reduced through the use of signal multiplexing, however, at the cost of increasing the time needed to access any particular data path and to transfer data and the increasing cost of such multiplexing circuitry and thus makes this approach of limited value. Moreover, the data transfer itself is slowed in the interface.
A similar problem exists when the connected data source requests access to the central processing equipment through an interrupt request, as such request places a burden on the system to quickly identify which data path and corresponding hardware element is making such request and to quickly provide the service of such an interrupt to the particular identified device and data path. Previously used interrupt techniques are ultimately limited by the increased number of data paths. The interrupt request circuitry commonly requires a separate signal for each board or path requesting interrupt service, which would then translate into a separate path for each board, which typically contains a limited number of interrupt request paths. Alternately, all circuit boards may share a single interrupt line with the central processor polling the device to determine the originator of the interrupt request. However, polling procedures slow the system by a significant factor which may impose a limitation on the size of the system. In another approach, the interrupt request may be daisy-chained from data paths organized on separate circuit boards; however, this requires specialized and expensive cables which must be installed in a particular manner, and provides slow interrupt service.
Therefore, the use of conventional techniques for addressing devices external to the processor and servicing interrupt requests becomes a significant limitation of the size and effectiveness of timing of data communication systems.